The present invention relates to a method of and apparatus for manufacturing ultrafine particle film and, more particularly, to a method of and apparatus for manufacturing ultrafine particle film of ultrafine particles such as of metals, oxides, nitrides, carbides and so forth, on a substrate at a good reproducibility, uniformly, effectively and efficiently.
The present inventors have made some proposals concerning the manufacture of ultrafine particle films. The characteristic features and practical problems of these proposed technics will be explained hereinunder.
For instance, Japanese Patent Laid-Open No. 28334/80 discloses an apparatus having a vessel for forming ultrafine particles. The space in the vessel is divided at least into an upper chamber and a lower chamber by means of a partition plate having an aperture which is adapted to be closed by means of a shutter mechanism. An evaporation source is disposed in the lower chamber while a substrate on which the ultrafine particles are to be deposited is disposed in the upper chamber. In this case, since the aperture of the partition plate is adapted to be opened and closed by means of a shutter, the ultrafine particles are prevented from coming from the lower chamber to the upper chamber when the shutter is kept closed. It is, therefore, possible to precisely control the amount of deposition of the ultrafine particle film. In addition, it is possible to obtain a good symmetry of the convection in the upper chamber because the ultrafine particles are supplied into the upper chamber through the aperture. Thus, this technique offers the advantage of uniform deposition of the ultrafine particles to the substrate, and can be suitably be used in the formation of an ultrafine metallic particle film. On the other hand, however, this technique imposes the following problem when the pressure of atmospheric gas for forming the ultrafine metallic particle film is low. Namely, to obtain the required thickness of the ultrafine particle film, it is necessary that the distance between the evaporation source and the substrate be sufficiently small. However, a too short distance between the evaporation source and the substrate causes the temperature of the substrate to rise excessively which deteriorates the film the result being an inferior performance of the ultrafine particle film.
The present inventors have proposed in Japanese Patent 151575/79 another apparatus in which gases in a vessel are excited and activated by a high-frequency electric field, and various vapors for forming the ultrafine particles are supplied to the atmosphere containing the activated gases, thereby to form various ultrafine particles. In this case, the gases in the vessel are excited and activated by a high-frequency electric field of 13.56 MHz applied to a high-frequency coil for an easier reaction with metal vapors, to facilitate various ultrafine particles such as oxides, nitrides, carbides and so forth. According to this technique, therefore, it is possible to easily form various ultrafine particles having low vapor pressure and high melting point such as oxides, nitrides, carbides and so forth, by evaporating a material having a low melting point and high vapor pressure, such as metals. As has been explained, this technique is useful for the manufacture of ultrafine particle films, but still suffers several practical problems as stated below.
First, it is to be pointed out that, since the high-frequency coil is wound at a comparatively low density, the vapor introduced into the chamber accomodating the gases excited by the high-frequency coil remains not only at the inside of the coil but is scattered also to be outside of the coil. This is attributable to the fact that the mean free path of the gas or vapor in the chamber is extremely short due to the high pressure in the chamber.
The above-explained point does not cause any substantial problem provided that the plasma is uniformly formed in the chamber. Generally, however, it is extremely difficult to form a uniform plasma in the chamber, although the state of plasma varies depending on various factors such as the size and shape of the high-frequency coil, gap between the walls defining the chamber and the high-frequency coil and so forth. In other words, it is extremely difficult to form uniform ultrafine particles, unless the plasma is formed uniformly. In addition, it is often experienced that some of the ultrafine particles formed in the chamber undesirably attach to the wall of the chamber. To avoid this, it is necessary to take practical countermeasures.
The second problem is that the apparatus is too complicated and expensive. This is caused by the fact that the ultrafine particles formed in the chamber and other ultrafine particles attaching to these particles are induced from the chamber of locally high by means of a D.C. electric field applied between the evaporation source and the substrate.
The third problem is that, since the coil itself is exposed to the high-frequency plasma, there is a possibility that the atoms released from the coil are retained as impurities by the ultrafine particle film, in which a high purity is desirable.
In regard to the formation of ultrafine particle films such as of oxides, nitrides, carbides and so forth, the technique disposed in Japanese Patent Laid-Open No. 28334/80 is rather inferior to the method disclosed in Japanese Patent Laid-Open No. 151575/79 in which the ultrafine particles are formed in an excited gaseous atmosphere, for the practical reasons shown below.
Namely, the former technique requires the additional step of heat treatment for controlling the composition of the ultrafine particles. In addition, the mean particle size of the ultrafine particles tends to be changed by the heat treatment. These reasons will be described in more detail in the description of the preferred embodiments.
The ultrafine particle films have wide application including atmospheric sensors for sensing humidity and gas, infrared ray sensors, laser power meters, selective absorption films for solar thermal conversion, photo-information storage films, catalysts and so forth.
Under these circumstances, the present inventors have proposed, in Japanese Patent Laid-Open No. 27925/80, a method of manufacturing an ulrafine particle film sensor consisting of ultrafine particles of oxides of Sn which is very useful as the material of gas sensors and humidity sensors. The essence of this proposal resides in a process having the steps of evaporating Sn or an oxide of Sn (SnO, SnO.sub.2) in an atmosphere of a gas pressure of between 0.1 and 1 Torr, and depositing the evaporated ultrafine particles on a substrate to form an ultrafine particle film of an Sn oxide thereby forming a sensitive body.
The prior art methods heretofore described are those which have various drawbacks although having various advantages as explained.